The present invention relates to an electronic component mounting method and apparatus for automatically mounting electronic components onto a board such as a circuit board or the like.
A common construction of a conventional electronic component mounting apparatus is explained by referring to FIG. 23. Reference numeral 31 denotes a rotating member 31 which is driven into intermittent rotation by motive power being transferred from a drive means 32 to an index device 34 via a speed reducer 33. Around the lower end of the rotating member 31, a plurality of suction nozzles 35 (35a, 35b, 35c, . . . ) are placed at equal intervals with an intermittent rotation pitch so as to be rotatable about their respective axes and also liftable. Each suction nozzle 35 is communicated with a sucking device (not shown) via a pipe 36 projecting upward from an axial center of the index device 34. Further, mechanical valves 37 for changing the suction force are attached on the suction nozzles 35, respectively. Thus the electronic component mounting apparatus is so constructed that the suction forces of the suction nozzles 35 to electronic components are changed when the component is sucked with the suction nozzle, when the sucked component is mounted on the board, and when the sucked component is discharged (suction releasing operation for faulty components).
Further, the rotation of an input shaft 38 directed to the index device 34 is transferred to a cam shaft 40 via a timing belt 39, so that a rod 42 moves up and down according to the shape of the cam 41 as the cam 41 rotates. When a suction switch 43 of the mechanical valve 37 is turned on with the rod 42 moving up and down, the suction forces of the suction nozzles 35 are increased so that electronic components are sucked.
Further, a similar cam mechanism is provided for each of mounting and discharge of electronic components. When a release switch 44 of the mechanical valve 37 is turned on, the suction forces to the electronic components are lowered so that the suction is released.
Reference numeral 45 denotes a component feed unit, where a plurality of component feeders 47 are arranged in parallel on a movable table 46 which is movable in the X direction with a drive means 48 comprising a motor or the like. A tape on which a large number of electronic components are accommodated in a row is wound around a reel 47a fitted to each component feeder 47, so that the electronic components accommodated on the tape are pulled out one by one to a component feed position 50. Then when the component feed position 50 of any component feeder 47 is positioned just under the suction nozzle 35, the suction nozzle 35 is lowered so that the electronic component 51 is sucked up and taken out.
The suction nozzle 35 that has sucked up the electronic component 51 moves up, and the sucked electronic component 51 is conveyed forward, as viewed in FIG. 23, by the rotation of the rotating member 31. During this conveying process, the electronic component 51 is subject to angle correction. The angle correction is intended to adjust the mounting angle of the electronic component 51 with respect to a circuit board 52.
The circuit board 52, on which the electronic component 51 is mounted, is horizontally supported on a board support base 53. Since an X-axis drive mechanism 54 and a Y-axis drive mechanism 55 are coupled with the board support base 53, the circuit board 52 can be moved and positioned to any location in a horizontal plane.
When the electronic component 51 is mounted on the circuit board 52, the electronic component mounting position on the circuit board 52 is positioned just under the suction nozzle 35. Then the lowered suction nozzle 35 releases the electronic component 51 from suction, so that the electronic component 51 is mounted on the circuit board 52. After that, the suction nozzle 35 moves up to return back.
Such a sequence of operations completes the mounting operation for one piece of electronic component. When a plurality of electronic components are mounted, the above mounting operation is repeatedly performed with respect to each electronic component in accordance with a previously specified mounting order of electronic-component-mounting positional information.
However, in the constitution of the electronic component mounting apparatus, the mechanical valves 37 are switched over under the control by the cam 41 interlocked with the periodic operation of the electronic component mounting apparatus. Since the mechanical valve 37 is switched over at the same timing, the following issue would arise due to changes in the speed of mounting operation.
The operation of the electronic component mounting apparatus is completed by an input shaft 40 turning to one rotation (0 to 360 degrees). Displacement in air pressure (vacuum pressure) due to the switching of the mechanical valve 37 with respect to angular displacement of the input shaft 40 in a sucking operation is shown in FIG. 24A. A curve X is the air pressure displacement curve while a curve Y is the nozzle height displacement curve. It is clear from FIG. 24A that a certain time is required since a switch of the mechanical valve 37 until an air pressure necessary for the suck-up of the electronic component is reached.
In order that the electronic component 51 is stably sucked up by the suction nozzle 35, an expression (1) must be satisfied among an angle .theta..sub.P of the input shaft 40 formed when the curve X in FIG. 24A reaches a suction-enable lower limit air pressure P.sub.S, a lower limit angle .theta..sub.L and an upper limit angle .theta..sub.U of the input shaft 40 formed while the curve y keeps at a zero nozzle height. It is noted that reference character T denotes a time required for the input shaft 40 to turn one rotation at a reference operational velocity of the present description. EQU .theta..sub.L &lt;.theta..sub.P &lt;.theta..sub.U ( 1)
However, when the operational velocity of the electronic component mounting apparatus is increased, the suction nozzle 35 starts to turn back upward before the suction force of the suction nozzle 35 reaches a suction-enable lower limit air pressure P.sub.S as shown in FIG. 24B. Thus, a relationship represented by an expression (2) results, where the relationship of the expression (1) is not satisfied such that the electronic component 51 could no longer be sucked: EQU .theta..sub.L &lt;.theta..sub.U &lt;.theta..sub.P ( 2)
Conversely, when the operational velocity of the electronic component mounting apparatus is lowered, the suction force of the suction nozzle 35 reaches the suction-enable lower limit air pressure P.sub.S before the suction nozzles 35 reach the suction position, as shown in FIG. 24C. Thus, a relationship represented by an expression (3) results, where the expression (1) could no longer be satisfied such that suction failures such as erect suction are more likely to occur: EQU .theta..sub.P &lt;.theta..sub.L &lt;.theta..sub.U ( 3)
As seen above, if the mechanical valve 37 is switched over at the same timing, the range of operational velocity is limited such that the suction stability deteriorates in high-speed and low-speed ranges. In addition, whereas the above explanation has been exemplified above by sucking operation, the case is similar also with the mounting operation, where the suction release timing in the high-speed and low-speed ranges becomes unstable such that the mounting precision would not be ensured and therefore mounting failures would be more likely to occur.